Prevalence of cardiac arrhythmias and R-R interval variation in healthy Thoroughbred horses during official Chuckwagon races and recovery.
Authors: Massie S L, Bezugley R J, McDonald K J, Leguillette R
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary Cardiac arrhythmias during high-intensity racing remain poorly characterised in competition Thoroughbreds, prompting researchers to examine their prevalence and significance in 82 clinically healthy horses competing in official Chuckwagon races, using electrocardiography during pre-race, racing, and recovery phases alongside measurements of plasma lactate and cardiac troponin. Despite the intensity of competition—confirmed by mean lactate levels of 28.5 mmol/L indicating supramaximal effort—arrhythmias were remarkably common, occurring in 92% of horses during racing (predominantly supraventricular premature complexes at 81% and ventricular premature complexes at 33%) and 58% during active recovery, though these were largely isolated events with no complex rhythms observed. Beat-to-beat R-R interval variation proved substantially greater during racing (−27.8 to +45.3%) compared with pre-race (−9.5 to +18.8%) and recovery phases, with statistically significant differences in both maximal shortening and lengthening (P < 0.0001), whereas all horses maintained normal cardiac troponin concentrations. These findings suggest that benign arrhythmias are a normal physiological response to extreme exercise intensity in healthy racehorses, though the clinical significance of the expanded R-R variability and the occasional occurrence of couplets and triplets during recovery warrant further investigation to establish safe versus concerning arrhythmia patterns in competition settings.
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Practical Takeaways
- •Isolated arrhythmias are a normal physiological response in healthy racehorses during high-intensity exercise and should not be automatically interpreted as pathological
- •The prevalence of exercise-induced arrhythmias is much higher than previously documented; veterinarians should expect to find arrhythmias in most racing Thoroughbreds during competition
- •Absence of elevated troponin confirms these arrhythmias do not cause myocardial damage, but clinical assessment should focus on complexity and context rather than simple presence of ectopic beats
Key Findings
- •92% of healthy Thoroughbreds exhibited arrhythmias during Chuckwagon racing, predominantly isolated supraventricular premature complexes (81%) and ventricular premature complexes (33%)
- •R-R interval variation was significantly greater during racing (−27.8 to +45.3%) compared to pre-race and recovery phases
- •No complex rhythms or cardiac damage (normal troponin levels) were observed despite high prevalence of arrhythmias during supramaximal exercise (lactate 28.5 ± 4.5 mmol/L)
- •Arrhythmias persisted into active recovery in 58% of horses but were predominantly benign isolated events